Few examples of variable rate transmission systems exist, particularly as to variable rate transmission systems in which no signals are transmitted to indicate the transmission rate to be used by a receiver for detecting the transmitted data. The present invention is concerned with providing a system and method for determining, from information obtained as a result of the decoding process, the transmission rate at which a received frame of data has been transmitted.
Variable rate transmission will now be described, with reference to FIG. 1. In the description which follows, the transmission rate of a given frame of data will be assumed to take values which vary among 1.2 kilobits per second (kbps), 2.4 kbps, 4.8 kbps, and 9.6 kbps. A transmitter selects one of the available transmission rates of 1.2 kbps, 2.4 kbps, 4.8 kbps, and 9.6 kbps in accordance with the quantity of data to be transmitted, and the characteristics of the transmission channel. The transmitter then transmits the data at the selected transmission rate over the channel.
FIG. 1 illustrates an example of a method for constructing a single frame transmitted in accordance with a variable rate transmission system. The illustration of the construction of a frame of data in FIG. 1 is intended to be exemplary, and not as indicating any particular constraints as to the manner in which a received frame of data is to be presented to the transmission rate judging system.
As illustrated, the frames as transmitted in a variable rate transmission system have a fixed time duration or "transmission frame width t.sub.f 181, regardless of the amount of data transmitted therein in accordance with a selected transmission rate. Use of such fixed width frames permits a receiver to handle communications on a frame by frame basis according to a uniform duration in time.
As illustrated in FIG. 1, in formatting the data for transmission according to different transmission rates, the duration or `width` of each transmitted bit is uniformly and proportionately varied. For example, if the width 182 of a bit of a frame formatted for transmission at the 9.6 kbps rate is one unit of time t, 182 two units of time 2t, 183, are needed per bit for transmission at the 4.8 kbps rate, four units of time 4t, 184, are needed for transmission at the 2.4 kbps rate, and eight units of time8t, 185, for transmission at the 1.2 kbps rate.
This results in a corresponding relationship for the instantaneous signal power of bits transmitted according to the different transmission rates. That is, since bits transmitted at the rate of 1.2 kbps are eight times longer in duration than bits transmitted at the 9.6 kbps rate, the bits transmitted at the 1.2 kbps rate can be transmitted at a correspondingly reduced power level, which can be, for example, one eighth of the power level at which bits are transmitted at the 9.6 kbps rate. The power level of bits transmitted at the 2.4 kbps and 4.8 kbps rates are also reduced correspondingly in relation to the power level used to transmit at 9.6 kbps. Reducing the transmitted power level for transmission at lower transmission rates, in this manner, reduces the overall transmitter power usage over time, and conserves energy (viz. battery power) in the transmitter, reduces interference with other transmitted communications, and increases the number of channels in the system which are available to be used at one time.
When a receiver has not been signalled with indication of the transmission rate of an incoming communication, the receiver must determine the transmission rate from the transmitted data itself. As illustrated in FIG. 1, when data is transmitted at rates lower than the maximum 9.6 kbps rate, data may appear to the receiver to have been transmitted repetitively. Thus, data which is transmitted at the 4.8 kbps rate appears to have been transmitted twice over the total transmission time in which each of the time intervals correspond to the time needed to transmit one bit at the 9.6 kbps rate. Thus, the method for determining the actual transmission rate of a transmission may be considered to be a method for determining the number of times that data bits have been repetitively transmitted.
As described above, the power levels at which data are transmitted vary according to different transmission rates. However, it is well known in digital communications that the bit error rate for a received transmission increases when the transmitted signal energy per bit of the transmission decreases. Thus, in order to reduce the probability of making an error in the determination of a transmitted data rate, a system is needed for determining the transmission rate of a received communication in a manner which accounts for increases in the bit error rate of the transmission due to the decreased signal energy which is present per transmitted bit for transmissions received at higher transmission rates.